The goals of this project are to evaluate a variety of factors which may influence the virulence of the pathogenic protozoa Entamoeba histolytica and Trichomonas vaginalis: An ongoing study on E. histolytica (in collaboration with LPD) is evaluating the roles of amebal viruses, selection pressures exerted by axenic cultivation and animal passage, and a serum-neuralizable amebal toxin in the variation in virulence among different amebal strains. A similar program of study Trichomonas vaginalis (in collaboration with Dr. B.M. Honigberg, Univ. Mass, Amherst) is in its early phases. We are developing virulence assays both in the newborn mouse and in tissue cultures. The program will be expanded to search different isolates for viruses, plasmids, other extrachromosomal elements or possible interaction with human wart viruses. Further characterization of amebal viruses has revealed that the beaded virus BV200 consists of 14 beads as the monomeric unit, incorporates labelled thymidine and excludes uridine and thus represents another DNA amebal virus.